The present invention relates to diamond workpieces and, more particularly, to their fabrication based on chemical vapor deposition technology. The diamond form of carbon possesses many desirable physical properties such as hardness, chemical inertness, infrared transparency, and excellent heat conductivity coupled with very high electrical resistivity. Its hardness and thermal properties are but two of the characteristics that make diamond useful in a variety of industrial components such as high velocity water jet nozzles, high velocity air/abrasive nozzles, and high velocity mixing tubes. Although the ability to synthesize diamond by high pressure/high temperature (HP/HT) techniques has been known for some time, the requirement of high pressure and high temperature has been a limitation, for example, in product configuration. Diamond workpieces with a complex three-dimensional configuration cannot be obtained by such processes without significant finishing or machining.
Efforts to produce diamond workpieces with a configuration which varies in three dimensions have been directed toward the growth of diamond at low pressures, where it is metastable. For example, to form three-dimensional diamond workpieces such as water jet nozzles, air/abrasive nozzles, and mixing tubes, a tube or wire is typically used as a growth substrate, and diamond growth on these substrates occurs on more than one plane. The substrate may be etched away to obtain a self-supporting article.
A number of methods are known for growing diamond coatings. One such method is disclosed in U.S. Pat. No. 4,707,384. Another method is disclosed by E. V. Spitsyn et al., "Vapor Growth of Diamond on Diamond and Other Surfaces," Journal of Crystal Growth 52, 219-226 (1981). Additional methods are disclosed in U.S. Pat. Nos. 4,486,286; 4,504,519; 4,645,977; and 4,707,384.
One of these techniques involves the use of a dilute mixture of hydrocarbon gas (typically methane) and hydrogen, wherein the hydrocarbon content usually is varied from about 0.1% to about 2.5% of the total volumetric flow. The gas is introduced via a quartz tube located just below a hot tungsten filament which is electrically heated to a temperature ranging from between about 1750.degree.-2400.degree. C. The gas mixture dissociates at the filament surface, and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate is heated to a temperature of about 500.degree.-1100.degree. C.
A second technique involves the imposition of a positive discharge to the foregoing filament process. The discharge serves to increase the nucleation density and growth rate and is believed to enhance formation of diamond in the form of a film, as opposed to discrete diamond particles. Of the plasma systems that have been utilized in this area, there are three basic systems: microwave plasma, RF (inductively or capacitively coupled) plasma, and DC plasma. The microwave and RF plasma systems utilize relatively complex and expensive equipment which usually requires complex tuning or matching networks to electrically couple electrical energy to the generated plasma. Additionally, the diamond growth rate offered by these two systems can be quite modest.
In general, processes for the chemical vapor deposition of diamond involves the selection of operating parameters such as the selection of a precursor gas and diluent gases, the mixture proportions of the gases, pressure and temperature of the gases, substrate temperature, and means of gas activation. These parameters are adjusted to provide diamond nucleation growth on the substrate. Mixture proportions and conditions must provide atomic hydrogen to stabilize the surface of the diamond film and preferably minimize the deposition of graphite. Codeposition of graphite is more evident if the hydrocarbon (methane) concentration is increased to above 3%.
Known CVD techniques provide diamond workpieces with a rough, chaotic surface typical of polynucleated crystalline material and exhibit a Raman line at 1332 cm.sup.-1. However, diamond workpieces such as high velocity water jet nozzles and high velocity air/abrasive nozzles require minimum surface defects and deviations in configuration to avoid breaking up the water/air jet. The nozzle used in a water jet is a free jet nozzle. Once the water stream leaves the top inner diameter edge of the nozzle, it does not contact the rest of the inner diameter of the nozzle because of the inward radial momentum of the water caused by the inward flow of water toward the nozzle in the reservoir. Consequently, the only critical part of the inner diameter is the corner edge at the top of the nozzle. This edge must have a radius of curvature of 1-3 .mu.m, and its included angle must be within 3.degree. of a 90.degree. angle. In addition, the nozzle aperture on this top edge must not be out of round by more than 0.2%. All of these specifications are necessary to produce a symmetric, stable jet stream which can travel 6" before Rayleigh instability breaks up the jet. Although smooth surfaces can be obtained for diamond workpieces which are grown on highly polished substrates, post-deposit abrasive polishing is often still required to provide a suitable component. Because the hardness of diamond varies by an order of magnitude with the crystallographic direction, it is difficult to polish polycrystalline diamond with a large grain size to the mirror-like finish demanded by the water jet nozzle application. Grains with a polishing-resistant orientation will stand above the surface, while grains with a soft orientation lie below the average surface level of the polish. This variability in polishing makes it very difficult to produce sharp inner diameter edges on the water jets with the specifications mentioned above. In addition, polishing large-grain diamond samples results in pull-out of the grains from the sample, which makes it more difficult to produce the sharp inner diameter edge required.
Workpieces produced by conventional CVD processes provide diamond deposits with ingrown stresses, which cause cracks and grain pull-out. Conventional diamond mixing tubes have failed because of circumferential and axial cracks generated by stress produced during the deposition process in the diamond walls of the mixing tube. When Al.sub.2 O.sub.3 is used as an abrasive, sections of the funnel and mixing tube walls, defined by these ingrown cracks, flake off and cause the abrasive jet to deviate from the straight line path through the mixing tube. The deflection of the jet can cause a catastrophic failure of the mixing tube. To counteract these problems, a metal shield is often placed around the diamond mixing tubes to impart compressive stress to take advantage of the greater mechanical strength of diamond in compression, as compared to tension. This has not proven to be a satisfactory solution. Therefore, it is desirable to provide diamond workpieces with limited ingrown stresses so as to provide greater mechanical strength and greater resistance to particle pull-out when polished.